1. Field of the Invention
This invention relates generally to a valve operating system of an internal combustion engine, and more particularly to improvements in a bearing structure for a camshaft.
2. Description of the Prior Art
A variety of valve operating systems have been hitherto proposed and put into practical use. In typical ones of these, intake and exhaust valves are arranged to open and close upon being driven directly by a camshaft or indirectly through a rocker arm, a swing arm or the like by a camshaft. Such a camshaft is formed with a plurality of bearing journal sections which are rotatably fitted respectively in the bores of bearing sections provided in a cylinder head of an internal combustion engine. Each bore is defined by a bearing surface to which the camshaft bearing journal section is slidably contactable. Each bearing surface and each camshaft bearing journal section is cylindrical such that the cross-section perpendicular to the axis of the bore is accurately circular. The bearing section bearing surface having a diameter slightly larger than that of the camshaft bearing journal section, thereby forming therebetween an oil clearance to be supplied with lubricating oil for forming an oil film.
However, the following difficulties have been encountered in the above-discussed valve operating systems: When an intake or exhaust valve is opened upon lift of a camshaft cam for any engine cylinder, the reaction of a valve spring is applied to the camshaft. When the lift of the camshaft cam is completed, such valve spring reaction which has applied to the camshaft disappears. Accordingly, if there is a larger oil clearance between the bearing section bearing surface and the peripheral surface of the camshaft bearing journal section, the camshaft largely displaces at various diametrical directions, so that the substantial amount of cam lift or lift characteristics of the intake or exhaust valve becomes inaccurate. As a result, in case of a multi-cylinder internal combustion engine, the opening and closing timings of the intake and exhaust valves are different in the respective engine cylinders, thereby deteriorating engine running stability during idling.
If the oil clearance is set extremely small to obtain a high engine running stability during idling, a sufficient oil film cannot be obtained between the bearing section bearing surface and the peripheral surface of the camshaft journal section, thereby failing good lubrication. This will lead to seizure and the like in the engine. Thus, the above-discussed conventional valve operating systems cannot meet the conflicating requirements of obtaining good lubrication and idling stability. This is particularly conspicuous in case the diameter of the camshaft journal section is considerably large.